Transmission system



April 19, 1932.

S.DOBA

TRANSMISS ION SYSTEI4 Filed April 12. 1930 3 Sheets-Sheet l /NVEA/mf? S.DoaA A T 70E/VD /Zf 4 56.2 ,d

April 19, 1932.

S. DOBA TRANSMI SS ION SYSTEM Filed April 12, 1930 3 Sheets-Sheet 2wwf/vm? S. DosA April 19, 1932. 5DOBA, 1,854,828

TRANSMISS ION SYSTEM Filed April 12, 1930 3 Sheets-Sheet 3 /m/fA/TURDOBA Patented Apr. 19, 1932 NlTED STATES TENT OFFICE STEPHEN DOBA, OFBROOKLYN, NEW YORK, ASSIG-NOR TO BELL TELEPHONE LABORA- TORIES,INCORPORATED, OF NEW YORK, N. Y., A CORPORATION F NEW YORK.

TRANSMISSION SYSTEM Application mea April 12',

This invention relates to signal transmission systems and particularlyto volume control circuits for governing the energy volume range atdifferent points on signal transmission systems.

One object of the invention is to provide a'transmission system withvolume control circuits that shall reduce the energy volume rangetransmitted over the lsystem a fixed per cent of the energy volume rangereceived for transmission.-

Another object of the invention is to provide a transmission system withvolume control circuits that shall reduce at the trans- 15. mitting endof the system the energy volume range transmitted over the system afixed. per cent of the energy volume range received for` transmissionand that shall restore the volume range transmitted over the systemtothe ggoriginal energy volume range at the receiving end of the system.

A further object of the invention is to pro- Vide a transmission linewith a vacuum tube loss device connected in series with the line a-t thetransmitting end thereof that shall reduce the'energy volumerangetransmitted over the line a fixed per cent of the energy volume rangereceived for transmission and a vacuum tube loss device connected acrossgo the line at the receiving end thereof that restores the transmittedenergy volume range to the original volumerange.

In radio broadcasting programs of speech or music, the program is veryoften transmitted for some distance over transmission lines beforebroadcasting; This is true where the broadcasting station and the studioare widely separated. The energy volume range or the speech or music maybe such as to overloa-d the apparatus on the transmission line.

The present invention provides means for reducingthe energy volumerangeof speech cr music at the transmitting endI of a line and forrestoring the original energy volume range at the receiving end of theline. Thus in broadcasting operations, where the studio is ata distancefrom the broadcasting sta` tion, the volume range of the program may bereduced when being transmitted from the 5u studio' to the broadcastingstation and. then 1930. Serial No. 443,640.

restored to the original range at the broadcasting station. The presentinvention also provides for reducing very wide energy volume ranges ofspeech or music to enable recording on films or phonograph records andfor restoring the original range upon reproduction from the film orphonograph record.

In accordance with the present invention one or more vacuum Vtube lossdevices are connected in series with a transmission line at thetransmitting end thereof for reduc-ing the volume rangeof the signalstransmitted over the line. At the' receiving end of the line one or morevacuum tube lossdevices are connected across the line and are socontrolled as to restore the volume range of the signals to theiroriginal range. At the transmitting end of the line the signals whichare received for transmission are assumed to have a relatively wideenergy volume range. Such signals at the transmitting station arereduced so that the volume range transmitted over the line has a certainper cent of fraction of the volume range of the signals as received fortransmission. At the receiving end .of the line the volume range of thesignals is restored to the original range received for transmission atthe transmitting end of the' line.

In a volume control system constructed in accordance with -the inventionone or more impedances which are in the form of vacuum tube ,lossdevices are connected in serieswith the transmission line and one ormore vacuum tube loss devices are connected in series with a branchcircuit joined to the transmission vline adjacent to and beyond the lastimpedance device in series therewith. A condenser is provided forcontrolling the potential on the grids of the vacuum tube'loss devicesin series with-the line and the grids of the vacuum tube loss devices inthe branch circuit. Means are provided for controlling the charge on thecondenser to insure a constant volume beyond the impedance device Vinthe 05 branch circuit. When constant volume is maintained beyond theimpedance device in the branch circuit, each of the impedance devices inseries with the main line reduces the volume range of the transmittedsignals 100 a fraction of or a per cent of the range ot the signals'received for transmission over the line.

At the receiving end of the transmission line one or more vacuum tubeloss devices are connected across the line and a branch circuit having avacuum tube loss device connected in series therewith is joined to theline adjacent `to and before the first impedance device at the receivingend of the line. The vacuum tube loss device in series with the branchcircuit and the devices connected across the line have their impedancescontrolled in accordance with the charge on a condenser. Thecondenservaries the potential impressed on the grids of the impedancedevices. The charge on the condenser is controlled by means joined tothe branch circuit beyond the vacuum tube loss device in seriestherewith. The charge. on the condenser is so varied as to maintainconstant energy volume on the branch circuit beyond the vacuum-tube lossdevice therein, and to control the vacuum tube loss devices connectedacross the line so as to restore the transmitted volume range to theoriginal volume range of the signals received for transmission at thevtransmitting end of the line.

ume control circuits at the transmitting end of the system. Y

Fig. 3 is a diagrammatic vlew of the circuits at the receiving end ofthe volume control system.

Fig. 4 isa modification of the vacuum tube loss device which may beinserted in series with the line at -the transmitting end thereof, andin series withthe branch circuits connected to the line.

.Referring to .'Figrl..acte-drawings, a" transmission -linegcomprisingconductors L1 and L2 is shown provided vwith a transmitting station Aand a receiving station B. At the transmitting station two vacuum tubeloss devices 1 and 2 are connected in. series with the line. A branchcircuit, comprising conductors 4 and 5 -a'n'd Yhaving a vacuum tube lossdevice V6 connected in 'series therewith,is` joined to theline beyondand adjacent to the im dance device 2. The potential-im resse Vonithegrids of the vacuum tube loss evices 1, 2 and 6 is governed inaccordance with the charge on a condenser 7. lhe charge on the condenserT gm-'orned by control means in accordance with the energy volume on thebranch circuit beyond the impedaneedevice G connected in seriez;therewith.

If three impedance devices are provided at the transmitting end ofaline, as is illustrated in Fig. 1 of the drawings, each of said im'-pedance devices will reduce the volume range one-third so that'constantvolume will be maintained on the branch circuit beyond the vacuum tubeimpedance device therein. The volume range beyond the first impedancedevice connected in series with the transmisA sion line will be reducedone-third of the range of the signals received for transmission. Thevolume range beyond the second lvacuum tube device in series with a linewill line. A detailed description of the loss devices 1, 2 and 6 will begiven when reference is made to Figs. 2 and 4.0i the drawings.

The term volume or energy volume7 used in the specification and claims1s assumed to be measured in transmission'units or decibels.

At the receiving end of the transmission i line two vacuum tube lossdevices 9 and 10 are assumed to be connected across the con-.

ductorsL1 and L2 of the transmissionline. A branch circuit comprisingconductors 11 and 12 and having,r a vacuum tube loss device 13 connectedin series therewithis joined to the transmission line adjacent to andbefore the loss device 9.l The loss'devices 9, 10 and 13 arecontrolledin accordance with the potential impressed upon the grids thereof bymeans of a condenser 14. The charge on the condenser 14 is governedby-control circuits 15 which are joined to the branch circuit beyond andadjacent to the vacuum tube loss device 13. The charge on the condenser14 is controlled to lmaintain a` constant volume range on the branchcircuit beyond the vacuurrltube loss device 13 and to control the lossldevices9 and 10 so as to restore the transmitted signals to theiroriginal volume range.

The vacuum tube loss devices 9 and 10operate in a manner opposite to thevacuum tube loss 'devices l and 2fso as to restore thev volume range ina. similarnianner to the reduction of the range at the transmitting end1n f series with the line in place of two loss devices as shown in Fig.1 of the drawings.

Similar parts in Fig. 2 of the drawings to those shown in Fig. 1 of thedrawings will be indicated by like reference characters The vacuum tubeloss device 1, which is connected in series with the line conductors Lland L2, comprises two three-element thermionic tubes 16 a'nd 17. The twotubes are connected in push-pull relationship, and the output circuitsthereof are connected to the input circuit of an amplifier tube 18. Theamplifier tube 18 is connected to a second amplifier tube 19 which isconnected to the line conductors L1 and L2 connecting the transmittingstation shown in Fig. 2 with the receiving station shown in Fig. 3 ofthe drawings.

The signals, which are to be transmitted over the line conductors L1 andL2, are received over conductors 20 and 21. Conductors 20 and 21 areconnected to the input circuits of the impedance tubes 16 and 17 bymeans 'of a transformer 22. Resistance and capacity elements areconnected across the secondary winding of the transformer 22 forimproving the frequency characteristic of the transmitted signals. Intheoutput circuits of the impedance tubes 16 and 17 are connectedresistance elements 23 which are joined to a battery 24 supplying platepotential to the tubes. The battery 24 also supplies plate potentialtothe'amplifier tubes 18 and 19. The impedance tubes 16 and 17 are jointedto the input circuit of the amplifier tube 18 by means of a transformer25 and a coupling resistance 26. The output circuit of the amplifiertube 18 is connected to the input circuit of the amplifier tube 19 bymeans of a coupling resistance 27. The output circuit of the amplifier'tube 19 is connected to the line conductors L1 and L2 by means of atransformer 28.

A Ibattery 29 is provided f'or supplying heating current to theimpedance tubes 16 and 17 and the amplifier tubes 18 and 19. Gridbiasing potential for the impedance tubes 16 and 17 is provided by thebattery 30 and grid biasing potential for the amplifier tubes 18 and 19is provided by the drop across resist? ance elements inthe heatingcircuit for the filaments of the tubes.

A branch circuit comprising conductors 4 and 5 is connected to thetransmission line beyond the impedance device 1 in series with the lineand has an impedance device 6 connected in series therewith.. Theimpedance device 6 comprises two three-element thermionic tubes 32 and33. The grid potential impressed on the impedance devices 32 and 33 inthe branch conductors 4 and 5 andupon the impedance devices 16 and 17 inthe main transmission line is controlled according to the chargeimpressed on a condenser 7. The charge impressed on the condenser 7 isgoverned by control circuits 8 comprising two amplifier tubes 34 and 35,two rectifier tubes 36 and 37 and two rectifier tubes 38 and 39. Thecondenser 7 is directly charged by the circuit including the battery 52and the output circuits of the tubes 36 and 37. The rectifier tubes 38and 39 control the operation of a relay 40 which governs the dischargeof the condenser 7.

A transformer 41 inserted in the branch conductors 4 and 5 is connectedto the input circuit of the impedance tubes 32 and 33. Resistance andcapacity elements are connected across the secondary winding of thetransformer 41 to improve the frequency characteristic of thetransmitted signals. A battery 42 is provided for supplying grid biasingpotential to the tubes k32 and 33. Resistance elements 43 in the outputcircuits of impedance tubes 32 and 33 are connected to the battery 24for supplying plate potential to the tubes. The amplifier tubes 34 and35 and the rectifier fube 39 are also supplied with plate potential bythe battery 24. A transformer 45 and coupling resistance element 46 areprovided for connecting the output circuits of the impedance tubes 32and 33 to the amplifier tube 34. A capacity element and a resistanceelement are connected in series with the former 45 'for improvmg thefrequency characteristic of the amplifying system. Similar resistanceand capacity elements are connected in series with the primary windingof the transformer 25 in the transmission line for a like purpose. Acoupling resistance 47 is provided between the output circuit of theamplifier tube 34 and amplifier tube 35. The amplifier tube 35 isconnected to the. rectifier tubes 36 and 37 by means of the transformer48. The input circuit of the rectifier tube 38 is connected to thetransformer 48 by means of a coupling resistance 49. The output circuitof the rectifier tube 38 is connected to the input circuit of therectifier tube 39 by means ofa coupling resistance 50 and a condenser51.

Plate potential for the rectifier tubes 36 and 37 is supplied by abattery 52 gnd plate potentialI forthe rectifier tube 38 is supplied bya battery 53. Heating current for the impedance tubes '32 and 33, theamplifier tubes 34 and 35 and the rectifier tubes 36'to 39, inclusive,is supplied by the battery 29.

Grid biasing potential for the rectifier tube 38 is supplied by thebattery 54.' The battery is provided for supplying grid biasing p0-tential to the tubes 36 and 37. Grid biasing potential is supplied tothe tubes'34 and 35 by the drop across resistance elements in thefilament heating circuit.

The relay 40 comprises an armature 57 which is adapted to engage acontact member 58 or a stop member 59,'an operating coil 60 and abiasing coil 61. The operating coil primary Winding of transics isincluded in the output circuit of the rectifier tube 39 The biasing coil61 is connected across the battery 24.l The coil 61 serves to hold thearmature 57 in engagement with the stop member 59 when the winding 60 isdeenergized. Normally, the current vfrom rectifier 39 through coil 60overcomes the effeet of the current through 61 so that armature 57 isheld in engagement with contact member 58. In this position of thearmature 57 the condenser 7 is discharged through a resistance element63. The coil 60 is only deenergized when the signals transmitted' overthe line conductors L1 and L2 and received through transformer 41fandtubes 32 and 33 are above a lower limiting value.v

When the signals transmitted over the line conductors L1 andjL2 andreceived through transformer 41 and tubes 32 and 33 are below the lowerlimiting value, the battery 54 impresses a strong negative potential onthe grid of the rectifier tube 38. This prevents any current fiowthrough the tube 38 and coupling resistance 50 between the tube 38 andthe tube 39 so that maximum Current flows through the winding 60. Thewinding 60, as heretofore set forth, moves the armature 57 intoengagement with the contact member 58.

lf the signals transmitted over the line conductors L1 and L2 are abovethe lower limiting value a potential is impressed on the grid of therectifier tube 38 from the secondary circuit of the transformer 48 whichpermits current fiow through the tube 38 and the coupling resistance 50.The potential across the coupling resistance 50' controls the grid ofthe rectifier tube 39 to prevent current fiow throughthe operatingwinding 60. The winding 61 operates the armature into engagement withthe stop-member 59 to insure against any discharge of the conf denser 7.

The impedance device 1 in series with the line conductors and impedancedevice 6 in series with rthe branch conductors 4 and 5;as shown in Fig.2 of the drawings, are controlled according to the charge on thecondenser 7. The charge on the condenser 7`is controlled according tothe output from the impedance tubes 32 and 33 in the branch conductors 4and 5. inasmuch as only one impedance device is connected in series withthe line conductors such impedance device will reduce the volume rangeof the transmitted signals one-half the range of said signals asreceived for transmission. The volume beyond the impedance device 6 inthe branch conductors 4 and 5 is held constant when the volume rangebeyond the impedance device l on the line is held one-half the volumerange of the signals before the impedance device in the transmissioncircuit.

The control circuit for the tube 16 from the condenser 7 extends fromthe grid of the 33 of the impedance device 6 in the branch circuit aretraced in like manner'through the grid biasing battery 42 and thesecondary winding of the transformer 41.

If so desired the impedance-device 1 in the transmission line and theimpedance device '6 in the branch circuit may be connected, as

shown in Fig. 4 of the drawings. In Fig. 4 of the drawings two impedancetubes 68 and 69 are shown with their anode-cathode cir-V cuits directlyconnected in series with the line,

which may b c tlic.-transmission line.. or the` branch circuit. Asimilarlimpedance device is disclosed in the abovementioned applicationof J. L. Hogg and S. Deba, lSerial No. 445,543.

Referring to the Fig. 3 of the drawings, a receiving station isillustrated which is provided with only one impedance device connectedacross the line conductors in place of two impedance devices as shown inFig. 1 of the drawings. Similar parts in Fig. 3 of the drawings to thoseshown in Figs. 1 and 2 of the drawings will be referred to by likereference characters.

The impedance device 9 Vcomprises two three-element thermionic tubes and76. The'anode-cathode circuits of the tubes 75 and 76 are connectedacross the line conductors Ll'and L2 between two transformers 77 and 78.Four resistance elements 79 are connected in the line adjacent to theconnection ofthe anode-cathode circuits of the tubes 75 and 76 acrossthe line. The resistance elements have a relatively high impedance ascompared to the impedance of the tubes. The secondary winding of thetransformer 78 is connected to the input circuit of an amplifier tube80. The amplifier tube 80 is joined to a second amplifier tube 81 byAcoupling resistance 82. The output circuit of the amplifier tube 81 isconnected to the transformer 83.

A branch circuit comprising conductors 11 and '12, similar to theconductors shown in Fig. 1 lof the drawings, is connected to the lineconductors L'1 and L2 adjacent to and before the impedance device 9. Animpedance device 13, which is similar in construction and operation tothe impedancerdcvice 6 shown in Fig. 2 of the drawings is con nected inseries Withthe branch'conductors 11 and 12. Control circuits 15 areprovided for governing the charge on ac-ondenser 14 to restore thevolume range of the signals transmitted over the line of their originalvolume range. The control circuits 15 are similar to the controlcircuit-8 shown in Fig. 2 of the drawings and a detailed descrpitionthereof is deemed unnecessary. A relay 40, similar to the relay 40 shownin Fig. 2 of the drawings is provided for controlling the discharge ofthe condenser 14. Condenser 14 is discharged in a manner similar to thedischarge of condenser 7. f

' Filament heating current for all tubes, shown in Fig. 3 of thedrawings is supplied from the battery 29. Plate potential for theimpedance tubes 75 and 7 6'and the amplifier tubes 80 and 81 is suppliedby the battery 24. Grid biasin potential for the impedance tubes 75 an76 is sup lied by a battery 85. The control circuit for t e grids of theimpedance tubes 7 5 and 76 extends from the grids of the tubes 75 and 76through the grid biasing battery 85, condenser 14 and ground to thefilaments of the tubes. The condenser 14 simultaneously controls thepotential impressed upon the grids of the impedance tubes 75 and 76connected across the line conductors and the impedance tubes connectedin series with the branch conductors 11 and 12. The impedance tubes inthe 'branch conductors 11 and 12 are so controlled by the condenser 14as to maintain a constant volume in the branch circuit beyond theimpedance tubes. Inasmuch as the impedance tubes 75 and 76 have theiranode-cathode circuits connected across the line conductors, an increase20 of the charge on the condenser will tend to /increase the negativepotential of the grids of the tubes 75 and 7 6 and reduce the impedancein the line caused by such tubes. The impedance device 9 will restorethe volume range of the transmitted signals to their original rangebefore reduction by the impedance device in series with the line at thetransmittin end thereof. A

Mo lfication in the system and in the arrangement and location parts maybe made within the spirit and scope of the invention and suchmodifications are intended to be covered by the appended claims.

What is claimed is:

1. In combination, a transmission 1ine, a variable loss device connectedin series wlth said line at the transmitting end thereof, andl meansconnected to said line beyond the loss device for governing the deviceto reduce the energy'volume range beyond the loss device a fixedfraction of the energy volume range before the loss device.

2. In combination, a transmission" line, a variable loss deviceconnected across said line at the receiving end thereof, and meansyconnected to said line before and adjacent to the loss device thereinfor governing the device to expand the energy volume range beyond thedevice'a fixed amount of the energy volume range before the impedancedevice.

3. In combination, a transmission line, a variable loss device connectedin series with said line at the transmittin end thereof, means connectedto said line eyond the loss device for governing the device to reducethe energy volume range beyond the loss device a fixed fraction of theenergy volume range before the device, a second impedance deviceconnected across the line at the receiving end thereof, and meansconnected to said line be` 7 fore and adjacent to the second impedancedevice -therein for governing the second impedance device to expand theenergy vol'ume range beyond the second impedance device a fixed amountof the energy volume range before the second impedance device.

' 4. In a signal transmission line, ymeans comprising a vacuum tube lossdevice at the transmitting end of the line for reducing the volume rangetransmitted over the line a xed 8 fraction of the energy volume rangebefore the loss device and means comprising a vacuum tube loss device atthe receiving end of the line for restoring the volume range of thereceived signals.

5. In a signal transmission system having signals of relatively widevolume range delivered thereto for transmission over a line, meanscomprising a vacuum tube loss device at the transmitting end of the linefor reduclng the volume rangea fixed per cent of the range of thesignals received for transmission and means comprising a vacuum tubeloss device at the receiving end of the line for restoring the volumerange of the signalt to the orlglnal range received at the transmittingend of the line. i

6. In a signal'transmission system having signals of relatively widevolume range delivered thereto for transmission over a line, 10 meanscomprising a vacuum tube loss device connected in series withthe line atthe trans-v mitting end thereof for'reducing the volume range of thesignals transmitted over the line and means comprising a vacuum tubeloss device connected across the line at the receiving end thereof forrestoring the volume range of the signals. f

7. In combination, a transmission line, a vacuum tube loss deviceconnected in series .with the line at the transmitting end thereof,means for controlling the potential impressed on the grid of said devicefor reducing the range of the transmitted signals, a vacuum tube lossdevice at the receiving end of the line connected across the line, andmeans for controlling the potentialimpressed on the grid of the deviceat the receiving end of the line for restoring the volume range of thesignals.

8. In combination, a transmission line, two three-element vacuum tubeloss devices connected in push-pullrelationshi and in series with saidline at the transmitting end thereof, a branch circuit connected te theline at n the transmitting end thereof, two three-element vacuum tubeloss devices connected in push-pull relationship and in series with saidbranch circuit a condenser for controlling the potential on the grids ofsaid devices according to the charge thereon, and means for controllingthe charge on said condenser accordlin to the energy volume on thebraneh circuit beyond the loss devicejthereln to reduce` the energyvolume range beyond the loss device in the line a fixed fraction of theenergy volume range before the loss device in the line.

9. In combination, a transmission line, two three-element vacuum tubeloss devices connected in push-pull relationship and across the line atthe receiving end thereof, a branch circuit' connected to the linebefore and adjacent to said loss device, two three-element vacuum tubeloss devices connected in push-pull relationship and in series with saidbranch circuit, a condenser for controlling the potential on the gridsof said devices according to the charge thereon, and means forcontrolling the charge on said condenser according to the .energy volumeon the branch circuit beyond the loss devices therein to expand theenergy volume range beyond the loss devices in the -line a fixed amountof the energyyolume range before the loss devices in the line.

10. In combination, a'transmission line, an impedance device connectedin series with said line at'the transmitting end thereof, a

vbranch circuit connected to the line beyond ,the impedance devicetherein, a second impedance device connected in series with the branchcircuit, and means governed according to the energy volume 1n the branchcircuit beyond the impedance devicetherein for controlling the impedancedevices to reduce-the volume range on` the line beyond the impedancedevice therein and to maintain constant volume range on the branchcirc-uit beyond the impedance device therein.

11. In combination, a transmission line, an impedance device connectedacross the line at the receiving end thereof, a branch circuit joined tothe line before and adjacent to said impedance device, a secondimpedance device connected in series with said branchV circuit, andmeans governed according to the energy volumev in the branch circuitbeyond the impedance device therein for controlling the impedancedevicesto expand the volume range on the line beyond the impedance devicetherein and to maintain constant volume ron the branch circuit beyondthe impedance device therein.

12. In. combination, a transmission line, a vacuum tube lossdeviceconnected in series with the line at the transmitting end thereof, abranch circuit connected to' the line beyond the loss device therein, `asecond vacuum tube loss` device connected in series with said branchcircuit, a condenser for controlling th e impedance of saiddevicesaccording to the charge thereon, and means joined to said branch circuitbeyond the device therein for controlling the charge on said condenserto maintain the volume range constant in the branch circuit beyond theloss device therein and to reduce the volume range on the line.

13. In combination, a transmission-line, a vacuum tube loss Adeviceconnected across the line at the receiving end thereof, a branch circuitjoined to the line before and adjacent to said loss device therein, asecond vacuum tube loss device connected in series with said branchcircuit, a condenser for varying the impedance of said loss devicesaccording to the charge thereon, and means joined to said branch circuitbeyond the loss device therein for controlling the charge on saidcondenser to maintain constant volume range in the branch circuit beyondthe loss device therein and to extend the volume range in the linebeyond the loss device therein.

14. In combination, a transmission line, a. vacuum t'ubefloss deviceconnected in series with the line at the transmitting end thereof asecond vacuum tube loss device connected in series with a branch circuitjoined to the line beyond said first mentioned loss device, a condenserfor varying theim edance of said devices according to the c argethereon, means joined to said branch line be ond the loss device thereinfor varying the c arge on the condenser to maint-ain the volume in thebranch line beyond the loss device constant and to reduce the volumerange in the line beyond the first mentioned loss device, a third vacuumtube loss device connected across the line at thereceiving endet/hereof,a second branch circuit joined to the line before andv adjacent to saidthird device therein, a fourth vacuum tube loss device connected inseries with said second branch circuit, a second con-v in the line atthe receiving end thereof to its original range.

15. In combination, a transmission line, an impedance device connectedin series with the line'at the transmitting end thereof, a branchcircuit connected to the line beyond vthe impedance device therein, asecond impedance device connected in series with the branch circuit,means governed according to the energy volume in the branch' circuitbeyond the impedance device therein for controlling the impedancedevices to reduce the volume range on the line beyond the impedancedevice therein and to maintain constant volume on the branch circuitbeyond the impedance device therein, a 'third impedance device connectedacross the -line at the receiving end thereof, asecond branch circuitjoined to the line before and adjacent to said third impedance device, afourth impedance device connected in series with said second branchcircuit, and means governed according to the energy volume in the secondbranch circuit beyond the fourth impedance device for controlling thethird and fourth impedance devices to expand the volume range onvtheline beyond the third impedance device therein and to maintain constantvolume on the second branch circuit beyond the impdance device therein.

In witness whereof, I hereunto subscribe my name this 11th day of April,1930.

STEPHEN DOBA.

